Nano-scale magnetic solid base catalyst and its preparatiion method

ABSTRACT

This invention describes a nano-scale magnetic solid base catalyst and its preparation method. The catalyst involves a magnetic core coated with a solid base active layer. The synthesis of the nano-scale magnetic cores was first carried out using a rapid nucleation method in a colloid mill reactor using a liquid-liquid reaction. The nano-scale magnetic cores were mixed with a mixed salt solution. The LDH-containing magnetic cores were prepared in a colloid mill reactor by a rapid nucleation method, and subsequently calcined to give a solid base mixed oxides coated on magnetic cores. The characteristics of this catalyst are: nano-scale, high surface area and high activity and selectivity in base-catalyzed reactions. The highly dispersed catalyst can be easily reclaimed using an external magnetic field because of its magnetism. The catalyst can be utilized in base catalysis in organic reactions such as glycol ether synthesis, ester exchange, aldol condensation, etc.

TECHNICAL FIELD

This invention describes nano-scale magnetic solid base catalysts and a method for their preparation.

BACKGROUND OF THE INVENTION

Current trends in catalysis research include the gradual replacement of homogeneous catalysts by heterogeneous catalysts. The advantages of using a solid base catalyst instead of a liquid base in catalytic reactions are:

(1) The high catalytic activity and selectivity and high purity of product;

(2) The catalysts can be easily separated;

(3) Preventing corrosion of equipment, reduction in effluent and more environmentally friendly.

However, heterogeneous catalysis systems usually have several shortcomings such as smaller reaction interface area and bigger transmission resistance, etc. These shortcomings significantly reduce the catalytic activity compared with homogeneous catalysis systems. There are also other reasons which limit the extensive application of heterogeneous catalysis systems.

Layered double hydroxides (LDHs) are a promising prospect in the field of heterogeneous catalysis. Shape-selective catalytic properties can be introduced by controlling the particle size and distribution and by adjusting the porosity. Nano-scale mixed metal oxides that have small particle size and large specific surface area have very efficient contact with the substrate, and give rise to high catalytic activities and excellent thermal stability. The disadvantage of these catalytic systems is the poor dispersion of the liquid-solid system, as well as difficulties in separation and reclamation of the catalyst. Furthermore, the large bed resistance caused by the small size of the catalyst particles in gas-solid catalysis systems leads to difficulties in their utilization in industry.

EP0421677A1 and EP0421678A1 relate to alkaline LDHs and calcined LDHs as solid base catalysts in glycol ether synthesis. LDHs can be represented by the general formula [M²⁺ _(a)M³⁺ _(b)(OH)_(9(2a+3b))][X]_(b), where M²⁺ and M³⁺ are divalent and trivalent metal cations, respectively; X is interlayer anion. LDHs were calcined to give Mg—Al—O compounds. Both of these works have not described how to solve the problems of dispersal, separation and reclaim of the catalyst however.

DESCRIPTION OF INVENTION

This invention describes a nano-scale magnetic solid base catalyst and its preparation method. This catalyst comprises a magnetic core coated by a solid base catalyst. This catalyst can be highly dispersed and easily reclaimed by using an external magnetic field because of its magnetism. The method of synthesis of this catalyst is as follows: Firstly, the synthesis of the nano-scale magnetic cores was carried out by a rapid nucleation method in a colloid mill reactor using a liquid-liquid reaction (as shown as the patent application 00132145.5). The nano-scale magnetic cores were mixed with corresponding mixed salt solution. Then, the LDH-magnetic core composite materials were prepared in a colloid mill reactor by a rapid nucleation method, and subsequently calcined to give a solid base mixed oxide compound. The mixed oxide compound is coated on the magnetic cores.

The nano-scale magnetic solid base catalyst in this invention has the formula: M-N—(O)/M′Fe₂O₄ Composition(wt %): M′Fe₂O₄: 15%-38%, M-N—(O): 85%-62%; wherein,

M is a divalent metal cation selected form the group consisting of Mg²⁺, Ni²⁺, Zn²⁺ Ca²⁺ or Co²⁺; N is a trivalent metal cation selected form the group consisting of Fe³⁺, V³⁺, Al³⁺ or Cr³⁺. M′ is a divalent metal cation selected form the group consisting of Mg²⁺, Cu²⁺, Ba²⁺, Ni²⁺, and M′ can be same as M or different;

M′Fe₂O₄ is a magnetic material. M-N—(O) is a composite oxide obtained

M′Fe₂O₄ is a magnetic material. M-N—(O) is a composite oxide obtained from calcination of the corresponding LDH coated on the surface of M′Fe₂O₄.

Preparation Method of the Catalyst:

(1) Preparation of nano-scale magnetic core M′Fe₂O₄

The mixed salt solution contained a soluble inorganic salt of M′(0.1-2.5 M) and a soluble Fe³⁺ inorganic salt (0.2-5.0 M) with the molar ratio of M′/Fe³⁺ of 0.2-5.0. The mixed alkali solution contained NaOH (0.1-5.0 M) and Na₂CO₃ (0.1-4.8 M).

The mixed salt and alkali solutions were simultaneously poured into a colloid mill reactor (as shown as the patent application 00132145.5). The rotation speed is 1000˜8000 rpm. The solutions were mixed thoroughly in the gap between the rotor and stator for 1˜8 min. The resulting slurry was transferred to an aging kettle and aged at 80˜120° C. for 2˜10 h. The solid product was filtered, washed, dried, and finally calcined at 800˜1000° C., affording the M′Fe₂O₄ powder. The particle size of M′Fe₂O₄ is within the range 20˜80 nm. The pH value of the mixed solution is adjusted in the range 8.5˜11.0 by addition of an alkali solution.

(2) Synthesis of the Magnetic Precursor M-N-LDH/M′Fe₂O₄

The mixed salt solution contained a soluble divalent inorganic metal salt M²⁺(X^(n−))_(2/n) (0.2-2.5 M) and a soluble trivalent inorganic metal salt N³⁺(X^(n−))_(3/n) (0.2-2.5 M) with the molar ratio of M²⁺/N³⁺ in range 2˜4. M′Fe₂O₄ powder was added to the mixed salt solution, such that the mass ratio of M′Fe₂O₄/divalent inorganic metal is in the range 1:17.1˜5.0, and stirred vigorously. The mixed alkali solution contained NaOH (0.1˜5.0 M) and Na₂CO₃ (0.1˜4.8 M).

The salt and alkali solutions were simultaneously poured into the colloid mill reactor. The rotation speed is 1000˜8000 rpm. The solutions were mixed thoroughly in the gap between the rotor and stator for 1-8 min. The resulting slurry was transferred to an aging kettle and aged at 80˜120° C. for 4˜10 h. The solid product was filtered, washed and dried, affording M-N-LDH/M′Fe₂O₄ containing the magnetic cores M′Fe₂O₄. The pH value of the mixed solution was adjusted by addition of alkali solution and kept in the range 8.5˜11.0.

(3) Preparation of M-N—(O)/M′Fe₂O₄ Catalyst

The M-N-LDH/M′Fe₂O₄ was calcined at 400˜600° C. for 2˜5 h with a heating rate of 10° C./min affording the magnetic solid base catalyst M-N—(O)/M′Fe₂O₄ with a particle size of 30˜100 nm.

The Fe³⁺ salt in step (1) could be: Fe₂(SO₄)₃, FeCl₃ or Fe(NO₃)₃. The M′ salt could be: M′SO₄, M′Cl₂ or M′(NO₃)₂. M′ could be Mg²⁺ or Ni₂₊.

The X described in step (2) could be: Cl⁻, SO₄ ²⁻, NO₃ ⁻ or CO₃ ²⁻. n is the valency of the anion X, n=1 or 2.

The preparation of magnetic cores and solid base catalyst is carried out by a rapid nucleation process in the colloid mill, in which the reactants mix and react thoroughly in a very short time, followed by a separate aging process for crystal growth. The resultant products are uniformly nano-scale. In the process of preparation of M-N-LDH/M′Fe₂O₄, it ensures the uniformity of coating structure because the crystal cores grow in the same preparation environment on the surface of the magnetic core. The particle size of this catalyst is in the range 30˜100 nm. Magnetic properties: coercive force: Hc=120˜108 A/m, saturation magnetization σ_(x)=2˜18 A/m. This catalyst can be highly dispersed and reclaimed by application of an external magnetic field because of its magnetism. The problems of reclaim and separation in other similar catalytic systems have been resolved.

Preferred Implements

EXAMPLE 1

(1) Preparation of Nano-Scale Magnetic Core MgFe₂O₄

The synthesis of the sample with Mg/Fe mol ratio of 1:2 was carried out as follows: one solution containing 8.46 g Mg(NO₃)₂.6H₂O and 27.07 g Fe(NO₃)₃.9H₂O in 130 ml of water and another containing 6.40 g NaOH and 14.20 g Na₂CO₃ were prepared. The two solutions were simultaneously poured into a colloid mild with the rotor rotating at 3000 rpm for 5 min. The resulting product was poured into an aging kettle and aged for 6 h with the temperature maintained at 100° C. The product was then repeatedly washed, filtered until the pH value of the washings was equal to 7, and finally dried at 70° C. for 24 h. The product was milled and calcined at 900° C. for 2 h in air. The heating rate was 10° C./min. Deionized water was used throughout all the experiments.

(2) Preparation of Magnetic Precursor MgAl-LDH/MgFe₂O₄

The synthesis of the sample with Mg/Al mol ratio of 3:1 was carried out as follows: one solution containing 38.46 g Mg(NO₃)₂.6H₂O, 18.78 g Al(NO₃)₃.9H₂O and MgFe₂O₄ in 130 ml of water with the Mg(NO₃)₂.6H₂O/MgFe₂O₄ mass ratio of 17.10, and another containing 6.40 g NaOH and 14.20 g Na₂CO₃ were prepared. The two solutions were simultaneously poured into a colloid mild with the rotor rotating at 3000 rpm for 5 min. The resulting product was transferred into an aging kettle and aged for 6 h with the temperature maintained at 100° C. The product was repeatedly washed, filtered until the pH value of the washings was equal to 7, and was finally dried for 24 h at 70° C. Deionized water was used throughout all the experiments.

(3) Preparation of Catalyst MgAl(O)/MgFe₂O₄

The magnetic precursor MgAl-LDH/MgFe₂O₄ was calcined at 600° C. for 2 h in air. The heating rate was 10° C./min. The particle size is in the range 30˜80 nm. The optimal particle size is 62 nm. Coercive force: Hc=120 A/m, saturation magnetization σ_(x)=2.1 A/m.

EXAMPLE 2

(1) Preparation of Nano-Scale Magnetic Core NiFe₂O₄

The synthesis of the sample with Ni/Fe mol ratio of 1:2 was carried out as follows: one solution containing 9.60 g Ni(NO₃)₂.6H₂O and 27.07 g Fe(NO₃)₃.9H₂O in 130 ml of water and another containing 8.00 g NaOH and 14.20 g Na₂CO₃ were prepared. The two solutions were simultaneously poured into a colloid mild with the rotor rotating at 5000 rpm for 3 min. The resulting product was transferred into a kettle and aged for 6 h with the temperature maintained at 100° C. The product was repeatedly washed, filtered until the pH value of the washings was equal to 7, and was finally dried for 24 h at 70° C. The product was then milled and calcined at 900° C. for 2 h in air. The heating rate was 10° C./min. Deionized water was used throughout all the experiments.

(2) Preparation of Magnetic Precursor ZnAl-LDH/NiFe₂O₄

The synthesis of the sample with Zn/Al mol ratio 2:1 was carried out as follows: one solution containing 39.56 g Zn(NO₃)₂.6H₂O, 25.13 g Al(NO₃)₃.9H₂O and NiFe₂O₄ in 130 ml of water with the Zn(NO₃)₂.6H₂O/NiFe₂O₄ mass ratio of 11.40 and another containing 14.4 g NaOH and 10.6 gNa₂CO₃ were prepared. The two solutions were simultaneously poured into a colloid mild with the rotor rotating at 5000 rpm for 3 min. The resulting product was transferred into an aging kettle and aged for 6 h with the temperature maintained at 100° C. The product was repeatedly washed, filtered until the pH value of the washings was equal to 7, and was finally dried for 24 h at 70° C. Deionized water was used throughout all the experiments.

(3) Preparation of Catalyst ZnAl(O)/NiFe₂O₄

The magnetic precursor ZnAl(O)/NiFe₂O₄ was calcined at 600° C. in air for 5 h. The heating rate was 10° C./min.

The particle size is in the range 30˜80 nm. The optimal particle size is 58 nm. Coercive force: Hc=110.5 A/m, saturation magnetization σ_(x)=3.5 A/m.

EXAMPLE 3

(1) Preparation of Nano-Scale Powder Magnetic Core NiFe₂O₄

The synthesis of the sample with Ni/Fe molar ratio of 1:2 was carried out as follows: one solution containing 9.60 g Ni(NO₃)₂.6H₂O and 27.07 g Fe(NO₃)₃.9H₂O in 130 ml of water and another containing 8.80 g NaOH and 10.65 g Na₂CO₃ were prepared. The two solutions were simultaneously poured into a colloid mild with the rotor rotating at 7000 rpm for 2 min. The resulting product was transferred to an aging kettle and aged for 6 h with the temperature maintained at 100° C. The product was repeatedly washed, filtered until the pH value of the washings was equal to 7, and was finally dried for 24 h at 70° C. The product was then milled and calcined at 900° C. in air for 2 h. The heating rate was 10° C./min. Deionized water was used throughout all the experiments.

(2) Preparation of Magnetic Precursor MgAl-LDH/NiFe₂O₄

The synthesis of the sample with Mg/Al mol ratio of 4:1 was carried out as follows: one solution containing 41.03 g Mg(NO₃)₂.6H₂O, 15.01 g Al(NO₃)₃.9H₂O and NiFe₂O₄ in 130 ml of water with the Mg(NO₃)₂.6H₂O/NiFe₂O₄ mass ratio of 5.00 and another containing 12.8 g NaOH and 8.48 g Na₂CO₃ were prepared. The two solutions were simultaneously poured into the colloid mild with the rotor rotating at 7000 rpm for 2 min. The resulting product was transferred into an aging kettle and aged for 6 h with the temperature maintained at 100° C. The product was repeatedly washed, filtered until the pH value of the washings was equal to 7, and was finally dried for 24 h at 70° C. Deionized water was used throughout all the experiments.

(3) Preparation of Catalyst MgAl(O)/NiFe₂O₄

The magnetic precursor MgAl-LDH/NiFe₂O₄ was calcined at 550° C. for 5 h in air. The heating rate was 10° C./min.

The particle size is in the range 35˜90 nm. The optimal particle size is 65 nm. Coercive force: Hc=130.2 A/m, saturation magnetization σ_(x)=2.6 A/m. 

1-3. (canceled)
 4. A nano-scale magnetic solid base catalyst having the formula: M-N—(O)/M′Fe₂O₄,wherein M′Fe₂O₄ is a magnetic material, and M-N—(O) is a composite oxide obtained from calcination of the corresponding layered double hydroxides coated on the surface of said M′Fe₂O₄; wherein M is a divalent metal cation selected from the group consisting of Mg²⁺, Ni²⁺, Zn²⁺, Ca²⁺ or Co²⁺; N is a trivalent metal cation selected from the group consisting of Fe³⁺, v³⁺, Al³⁺ or Cr³⁺; and M′ is a divalent metal cation selected from the group consisting of Mg²⁺, Cu²⁺, Ba²⁺, or Ni²⁺, and said M′ can be same as said M or different; and wherein M′Fe₂O₄comprises about 15 wt. % to about 38 wt. %, and M-N—(O) comprises about 62 wt. % to about 85 wt. %, of the total weight of said nano-scale magnetic solid base catalyst.
 5. A method for synthesizing nano-scale magnetic solid base catalyst according to claim 1, comprising the steps of: a) synthesizing of said nano-scale magnetic cores comprising the steps of: simultaneously adding mixed salt and mixed alkali solutions to a colloid mill reactor with the rotation speed at about 1000 rpm to about 8000 rpm, wherein said colloid mill reactor has a rotor and stator, and a gap therebetween; mixing said mixed salt and said mixed alkali solutions completely in the gap between the rotor and stator of said colloid mill reactor for about 1 to about 8 min to produce a slurry; pouring said slurry into an aging kettle and aging said slurry at about 80° C. to about 120° C. for about 2 to about 10 hours, resulting in a solid product; filtering, washing, and drying said solid product; and calcining said solid product at about 800° C. to about 1000° C., producing said nano-scale magnetic cores with the particle size about 20 nm to about 80 nm; wherein said mixed salt solution comprises soluble inorganic salts with about 0.1 M to about 2.5 M of said M′ and about 0.2 M to about 5.0 M of soluble Fe³⁺ inorganic salt such that the molar ratio of M′/said Fe³⁺ inorganic salt is between 0.2 to 5.0; wherein said mixed alkali solution comprises about 0.1 M to about 5.0 M of NaOH and about 0.1 M to about 4.8 M of Na₂CO₃; and wherein the pH of said mixed alkali solution is about 8.5 to about 11 and is adjusted by adding an alkali solution; b) synthesizing magnetic precursor M-N-LDH/M′Fe₂O₄ comprising steps of: simultaneously pouring mixed salt and mixed alkali solution into said colloid mill reactor with the rotation speed of about 1000 to about 8000 rpm; mixing said mixed salt and mixed alkali solutions completely in the gap between the rotor and stator of said colloid mill reactor for about 1 to about 8 min to produce slurry; pouring said slurry into an aging kettle and aging said slurry at about 80°C. to about 120° C. for about 4 to about 10 hours, resulting in a solid product; filtering, washing repeatedly, and drying said solid product, resulting in said M-N-LDH/M′Fe₂O₄ that contains said nano-scale magnetic cores; wherein said mixed salt solution has about 0.2 M to about 2.5 M of a soluble divalent inorganic metal salt M²⁺,(X^(n−))_(2/n) and about 0.2 M to about 2.5 M of soluble trivalent inorganic metal salt N³⁺(X^(n−))_(3/n), and said M′Fe₂O₄; wherein n is the valence of anions X, and n is 1 or 2; wherein the molar ratio of M²⁺/N³⁺ is about 2 to about 4; and the mass ratio of said M′Fe₂O₄/said divalent inorganic metal cation is about 1:5.0 to 17.1; and wherein said mixed alkali solution has about 0.1 M to about 5.0 M of NaOH and about 0.1 M to about 4.8 M of Na₂CO₃. c) preparing said solid base catalyst M-N—(O)/M′Fe₂O₄ wherein said M-N-LDH/M′Fe₂O₄ is calcined at about 400° C. to about 600° C. for about 2 to about 5 hr. with a heating rate of about 100° C./min, resulting said magnetic solid base catalyst M-N—(O)/M M′Fe₂O₄ in the particle size about 30 nm to about 100 nm.
 6. The method for synthesizing nano-scale magnetic solid base catalysts according to claim 5, wherein said Fe³⁺ inorganic salt is selected from the group consisting of Fe₂(SO₄)₃, FeCl₃ or Fe(NO₃)₃, said M′ is selected from the group consisting of M′SO₄, M′Cl₂ or M′(NO₃)₂; M′ is Mg²⁺ or Ni²⁺; and said X is Cl⁻, SO₄ ²⁻, NO₃ ⁻ or CO₃ ²⁻. 